Interface circuits are important for converting signals between systems that have different signal requirements (e.g., a first integrated circuit that has a first set of signal requirements and a second integrated circuit that has a second set of signal requirements). These interface circuits can be incorporated in the first IC as a last output stage, in the second IC as a first input stage, or as a stand-alone circuit that is interposed between the first IC and the second IC.
One trend in electronic systems is the reduction in the power supply voltage (e.g., Vcc). These systems that operate with lower power supply voltages are especially conducive for low-power consumption applications (e.g., portable electronic devices) since a lower power supply voltage leads to lower power consumption. However, as the supply voltage in systems is reduced, it becomes more of a challenge to meet certain electrical specifications. For example, certain requirements, such as voltage swing and common mode voltage, are increasing difficult to meet especially as the supply voltage for electronic systems decreases.
One example, of a specification with strict electrical requirements is the InfiniBand™ Architecture Specification. InfiniBand Trade Association publishes InfiniBand™ Architecture Specification Release 1.1, Vol. 1 and Vol. 2 (released Nov. 6, 2002) (see http://www.infinibandta.org/specs/register/publicspec/). This specification sets forth various parameters and requirements for the electrical, optical, mechanical specifications for use by designers of products and components that are compliant with the InfiniBand™ Architecture.
Interface circuits typically employ a wave-shaping or conditioning circuit to generate an output waveform to drive the next stage with signals that conform to the requirements of the next stage (e.g., a system compliant with the InfiniBand™ Architecture Specification). One important function of such a wave-shaping circuit is to shift the voltage level of the signals. In other words, shifting the voltage level is an important aspect in the design of the wave shaping or conditioning circuit. There are different approaches to shifting the voltage level in a circuit.
One approach is described in U.S. Pat. No. 4,713,560 that is entitled, “Switched impedance emitter coupled logic gate” (Inventor: William H. Herndon). U.S. Pat. No. 4,713,560 is directed to a circuit that uses emitter followers to level shift from the collectors of a differential pair. FETs that are connected at the differential pair collector nodes are used to adjust the output impedance. Unfortunately, one disadvantage of this approach is that it is difficult to control the output impedance by using emitter/source followers. Another disadvantage of this approach is that the output impedance of emitter/source followers is typically small, which can pose a problem for those applications that require a larger output impedance for the next stage.
A second approach is described in U.S. Pat. No. 4,999,519 that is entitled, “Semiconductor circuit with low power consumption having emitter-coupled logic or differential amplifier” (Inventors: Goro Kitsukawaet al.). U.S. Pat. No. 4,999,519 is directed to a circuit that also uses emitter followers to level shift from the collectors of a differential pair. FETs and resistors that are connected at the differential pair collector nodes are either OFF or operating in the linear region to disable or enable the emitter follower input.
Unfortunately, as noted earlier, one disadvantage of an approach that employs emitter/source followers is that it is difficult to control the output impedance by using emitter/source followers. Furthermore, as noted earlier, the output impedance of emitter/source followers is typically small, which can pose a problem for those applications that require a larger output impedance for the next stage.
A resistor can be coupled in series with the output port in order to increase the output impedance. However, adding the resistor consumes voltage swing, thereby decreasing the maximum output swing, which for certain applications is undesirable.
Another approach is described in U.S. Pat. No. 6,255,857 entitled, “Signal level shifting circuits” (Inventor: Stepan Iliasevitch). This approach employs stacked diodes to shift down the signal. One disadvantage of this approach is that the circuit can only shift the voltage in integer multiples of the diode turn-on voltage. Moreover, this approach does not appear to be designed to support systems with a low power supply voltage.
Yet another approach is to use source followers operating at class AB to level shift a signal. An example of this approach is described in a paper entitled, “A Low Voltage, Rail-to-Rail, Class AB CMOS Amplifier With High Drive and Low Output Impedance Characteristics,” Gabriel A. Rincon-Mora and Richard Stair, IEEE Transactions on Circuits and Systems—II: Analog and Digital Signal Processing, Vol. 48, No. 8, pages 753 to 761, August 2001. One disadvantage of this approach is that for low voltage applications (e.g., applications with a low supply voltage, VCC), the source follower does not provide a 50 ohm output impedance and a large voltage swing. Furthermore, the class AB CMOS amplifier level shifts the input common mode voltage of the output stage and not the common mode output voltage, which is required in some applications.
Another approach is to use cross-coupled pFET to level shift a signal. A first example of this approach is described in a paper entitled, “A Versatile 3.3/2.5/1.8-V I/O Driver Built in a 0.2-μm, 3.5-nm Tox, 1.8-V CMOS Technology,” Hector Sanchez, Joshua Siegel, Carmine Nicoletta, James P. Nissen and Jose Alvarez, IEEE Journal of Solid State Circuits, Vol. 34, No. 11, pages 1501 to 1511, November 1999. A second example of this approach is described in a paper entitled, “A Versatile 3.3/2.5/1.8-V I/O Driver Built in a 0.2-μm, 3.5-nm Tox, 1.8-V CMOS Technology,” Wen-Tai Wang, Ming-Dou Ker, Mi-Chang Chiang and Chung-Hui Chen, 2001 International Symposium on VLSI Technology, Systems, and Applications, Proceedings of Technical Papers, pages 307 to 310, 2001.
One disadvantage of this approach is that the approach is directed to single-ended applications (e.g., digital circuits). Unfortunately, this approach is not suitable for differential signal applications.
Based on the foregoing, there remains a need for a wave shaping circuit that overcomes the disadvantages set forth previously.